1. Field of the Invention
The present invention relates to audio processing, and, in particular, to adjusting the frequency response of microphone arrays to provide a desired response.
2. Description of the Related Art
Speech signal acquisition in noisy environments is a challenging problem. For applications like speech recognition, teleconferencing, or hands-free human-machine interfacing, high signal-to-noise ratio at the microphone output is a prerequisite in order to obtain acceptable results from any algorithm trying to extract a speech signal from noise-contaminated signals. Because of possibly changing acoustical environments and varying position of the talker with respect to the microphone, conventional fixed directional microphones (i.e., dipole or cardioid elements) are often not able to deliver sufficient performance in terms of signal-to-noise ratio. For that reason, work has been done in the field of electronically steerable microphone arrays operating under farfield conditions (see, e.g., Flanagan, J. L., Berkley, D. A., Elko, G. W., West, J. E., and Sondhi, M. M., “Autodirective microphone systems,” Acoustica, vol. 73, pp. 58–71, 1991, and Kellermann, W., “A self-steering digital microphone array,” IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Toronto, Canada, 1991), i.e., where the distance between a signal source and an array is much greater than the geometric dimensions of the array.
However, under extreme acoustical environments, which can be found, for example, in a cockpit of an airplane, only close-talking microphones (nearfield operation) can be used to ensure satisfactory communication conditions. A way of exceeding the performance of conventional microphone technology used for close-talking applications is to use close-talking differential microphone arrays (CTMAs) that inherently provide farfield noise attenuation. If the CTMA is positioned appropriately, the signal-to-noise ratio gain for the CTMA will be inversely proportional to frequency to the power of the number of zero-order (omnidirectional) elements in the array minus one. One issue of using differential microphones in close-talking applications is that they have to be placed as close to the mouth as possible to exploit the nearfield properties of the acoustic field. However, the frequency response and output level of a CTMA depend heavily on the position of the array relative to the talker's mouth. As the array is moved away from the mouth, the output signal becomes progressively highpassed and significantly lower in level. In practice, people using close-talking microphones tend to use them at suboptimal positions, e.g., far away from the mouth. This will degrade the performance of a CTMA.